NOTE:The
following proposal was submitted to and passed by the North American
Classification Committee and is posted here, with minor editing, with the
permission of the authors.This
proposed split only affects us indirectly in terms of classification by
truncating the range of B. nitidus,
but it does affect directly our English name usage (change to Gray-lined Hawk).

Description of
the problem:

Gray
Hawk (Buteo nitidus, hereafter B. n. nitidus) was described by Latham
(1790) as Falco nitidus, based on a
specimen from Cayenne, French Guiana.Subsequently, a new taxon based on a specimen from Veracruz, Mexico, was
described by Schlegel (1862) as Asturina
plagiata (hereafter B. n. plagiatus).Schlegel considered B. n. plagiatus a separate species from B. n. nitidus because it was larger, had more robust tarsi and
feet, and had a greater number of tail bands.In their review of North American birds,
Baird et al. (1874) concluded the two taxa were climatic races of the same
species, and this view has largely prevailed since (Bierregaard 1994,
Ferguson-Lees and Christie 2001, AOU 2010).However, not all ornithologists agree
with this treatment.Miller and
Griscom (1921), van Rossem (1930), and Sibley and Monroe (1990) treated the
taxa as distinct species.Friedmann
(1950), Stresemann and Amadon (1979), and the AOU (1983) treated them as
conspecific, but commented that the taxa might be full species.Johnson and Peeters (1963), in their
detailed analysis of plumage variation of woodland hawks, concluded, “striking
differences between the [northern and southern] races [of Gray Hawk] are
obvious”.These authors noted that
the plumage discontinuity occurs coincident with a gap in the species’
distribution in Costa Rica, which is also described by Stiles and Skutch
(1989).Blake (1977) presented
measurement data for all recognized subspecies of Gray Hawk, but he did not
quantitatively analyze measurement differences between taxa; he treated them as
conspecific, but noted that many consider them separate species.Millsap (1986) and Riesing et al. (2003)
evaluated morphological and genetic differences between the two taxa,
respectively, and concluded that they differed markedly.

New
information:

Until
recently there was no comprehensive published analysis of plumage, measurement,
and vocal data for the Gray Hawk on which to base a decision regarding the
species-level status of the two taxa (Banks et al. 2006).In a recent paper, Millsap et al (2011)
compared plumages, morphology, and vocalizations of B. n. nitidus and B. n.
plagiatus, and found that allopatric B.
n. nitidus and B. n. plagiatus
differ diagnosably at very high probability levels in all age and sex classes
across a range of plumage, measurement, and vocalization characters.Adjacent B. n. nitidus and B. n.
plagiatus populations were entirely separable based on plumage, even where
ranges of the two taxa approach one another in Costa Rica.Discriminant function analysis (DFA)
using measurements of body and tail pattern characters of 405 museum specimens
resulted in correct classification of > 98% of juveniles and adult
males and 88% of adult females, and DFA using alarm call measurements resulted
in correct classification of 100% of the vocalizations.These results parallel findings by Riesing
et al. (2003) that the mtDNA difference between the two taxa is on the order of
9%.

In
addition, we propose to change the distribution description for B. plagiatus to reflect regular
occurrence in southern New Mexico in the breeding season, based on Williams and
Krueper (2008), and to reflect occurrence of B. nitidus to 1600 m elevation based on Hilty (2003).

Recommendation:

Collectively,
all lines of evidence strongly suggest that the current conspecific treatment
of B. n. nitidus and B. n. plagiatus does not accurately
reflect the extent of differentiation between the two taxa.We recommend they be
considered two species as described below:

Distribution.—Resident from southern Arizona (rarely), southern New Mexico
(rarely), western (rarely) and southern Texas south through Middle America
(including the Bay Islands, off Honduras) to northwestern Costa Rica (Gulf of
Nicoya region).Northernmost
breeding populations in Arizona, New Mexico, and western Texas are largely
migratory southward in nonbreeding season.

Distribution.—Resident from Costa Rica (except northwest), Panama, Colombia,
Venezuela, Tobago, Trinidad, and the Guianas south, west of the Andes to
western Ecuador and east of the Andes to northern Argentina, Paraguay, and
southern Brazil.

Literature
cited:

American Ornithologists’ Union.1983.Checklist of North American Birds.6th ed.American Ornithologists’ Union,
Washington, D.C.

American Ornithologists’ Union.[Online].2010.Check-list of North American Birds.American Ornithologists’ Union,
Washington, D. C. <http://www.aou.org/checklist/index.php3> (24 February 2010).